How Much Does an ADU Cost in Seattle & King County? 2026 Price Guide

2026 ADU cost ranges by type

- Garage conversion: $100,000–$250,000. The cheapest path to a legal ADU because the slab, walls, and roof already exist. The price swings on plumbing runs, ceiling height corrections, and insulation upgrades.

- Attached ADU (basement or addition): $150,000–$300,000. Shares walls and usually utilities with the main house, which keeps sitework down.

- Detached ADU / backyard cottage (DADU): $250,000–$450,000+. A ground-up small home with its own foundation, envelope, and utility connections — and the strongest rents and resale value.

What's inside those numbers

Roughly speaking, a King County DADU budget breaks down into sitework and foundation, the building shell, mechanical/electrical/plumbing, interior finishes, and design + permitting. Two line items surprise homeowners most: utility connections (water, sewer or septic, power — anywhere from $10,000 to $50,000+ depending on distance and capacity) and site access (sloped or tight backyards cost more to build in).

Permit costs and timelines in 2026

Plan on roughly $5,000–$20,000 in permit and review fees depending on the city, plus design and engineering. Since Washington's HB 1337, most urban King County lots can add at least two ADUs, Seattle has no owner-occupancy requirement, and no extra parking is required in most zones. Permitting typically takes 2–4 months; construction adds 4–8 months for conversions and attached units, or 5–9 months for a new DADU. Start to finish, budget 8–14 months.

What an ADU earns back

Well-built ADUs near Seattle and Bellevue commonly rent long-term for $1,800–$3,200+ per month depending on size and location — Renton and south King County toward the lower end, Bellevue, Mercer Island, and central Seattle toward the top. Appraisers also increasingly credit finished ADUs toward resale value, which is why many of our clients treat a DADU as a long-term investment rather than an expense.

How to keep your ADU on budget

1. Verify zoning and utilities before design — we do this on every lot before drawings begin.

2. Choose a fixed-price contract. Blue Lake builds ADUs on a fixed-price, fixed-duration contract, so the number you sign is the number you pay.

3. Pick your finish level early. Cabinets, tile, and appliances move a DADU budget by $30,000+ either way.

4. Don't skip the small stuff that renters value: sound insulation, in-unit laundry, and a separate entrance path.

City-by-city starting points

See our local ADU pages for zoning notes and neighborhood-specific guidance: Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Renton, and Mercer Island.

Ready for a real number for your lot? Request a free consultation — we'll walk the site, check zoning and utilities, and price your ADU as a fixed bid.